Apparatus for brazing super alloys and refractory metals



Dec. 27, 1966 w. T. KAARLELA 3,294,949

APPARATUS FOR BRAZING SUPER ALLOYS AND REFRACTORY METALS Filed June 16, 1964 2 Sheets-Sheet l 5 WILLIAM T. KAARLELA ATTORNEY Dec. 27, 1966 w. 'r. KAARLELA APPARATUS FOR BRAZING SUPER ALLOYS AND REFRACTORY METALS 2 Sheets-Sheet 2 Filed June 16, 1964 WILLIAM T. KAARLELA INVENTOR.

ATTORNEY United States Patent Ofifice 3,2943% Patented Dec. 27, 1966 3,294,949 APPARATUS FOR BRAZING SUPER ALLOYS AND REFRACTORY METALS William T. Kaarlela, Fort Worth, Tex., assignor to General Dynamics Corporation, Fort Worth, Tex., a corporation of Delaware Filed June 16, 1964, Ser. No. 375,514 5 Claims. (Cl. 219-85) The present invention relates generally to metal joining.

More particularly, the invention pertains to a metal joining process and apparatus for refractory metals and super alloys.

The present invention is characterized by extremely brief heating and cooling cycles wherein metal joining techniques requiring various temperature-pressure relationships may be performed upon materials such as refractory metals, super alloys and the like, While at the same time providing such materials with a controlled environment, thus permitting joining of these materials to be effected without detrimental metallurgical changes occuring therein.

The aerospace industry, in particular, is confronted with the need for structural materials comprised of refractory metals and super alloys for use in present and future vehicles. This is due primarily to the inherent strength and stability at ultra-high temperatures of such metals. With the growing utilization of refractory metals a corresponding problem has arisen in the need for efficient processing of components fabricated therefrom. This problem is due to phenomena associated with refractory metals which causes them to recrystallize when subjected to the temperatures necessary to effect joining or annealing.

This recrystallization and its associated grain growth permanently and adversely affect the mechanical properties of refractory metal. Therefore, in the processing of such materials prime consideration must be given the relationships of temperatures and dwell times to which the material must be subjected in order to effect joining. These relationships (time and temperature) are critical since recrystallization and grain growth begin to occur in the object material immediately upon its entering the recrystallization zone. The degree of recrystallization is directly proportional to dwell time and to the amount by which the initial recrystallization temperature is ex ceeded, and thus the total temperature gradient experienced.

To the present time the radiant, vacuum-type furnace has usually been employed to join high temperature resistant super alloy or refractory type metal members.

Inasmuch as this type furnace effectively produces only radiant heat, it has an exceedingly poor rate of heat transfer. This is due to the relatively wide spacial disposition of the tantalum heaters used therein, in relation to the object workpiece as well as to a lack of transmitting atmosphere. Such furnaces therefore have not been capable of satisfactorily brazing refractory metals and super alloy metals. Because of the exceptionally high current required, the state-of-the-art heaters employed cannot be closely spaced without engendering direct arcing or shorting between heater and workpiece.

In the operation of such a conventional radiant heat vacuum furnace, the support fixture and insulation masses must also be substantially saturated with heat along with the mass of the workpiece because of the above mentioned widely spaced relationship of heaters and work. Obviously, the aggregate of masses of all these elements constitutes a large mass, which, in view of an extremely inefficient heat transfer gradient, results in a heating rate that is exceptionally slow and inefficient.

In the present invention, shorting is precluded even though the tantalum heaters employed are disposed in much closer proximity to each other than has heretofore been possible. Thus an increased concentration of heaters within the work area vastly increases efficiency and permits the brazing temperature to be driven much higher during any given time period.

With the brazing temperature substantially above the point at which recrystallization begins, it is obvious that the time required to achieve the brazing temperaturetime within the recrystallization zone-is extremely critical, and that prior furnaces having inherently slow and inefiicient heating rates destroy the intrinsic strength and heat resistant qualities of the material upon which they were designed to operate.

It has been demonstrated that time within the recrystallization zone is extremely detrimental to various materials and that great inefiiciencies result as an effect of the slow heating rate. However, a slow cooling rate is obviously equally as detrimental. Therefore, any process must be evaluated by the total time it requires within this zone, which determines the extent of joint degradation that is ultimately sustained by the materials involved. As hereinabove mentioned, all interior components of the conventional furnace and the workpiece must be saturated with heat to attain the metal bond. This results in substantial residual heat at the termination heating cycle. This residual heat dictates the necessity for an extremely long cooling cycle, thus further lengthening the time that the object material is sustained above its recrystallization point.

Another deleterious characteristic of the prior devices and their resultant methods resides in the high initial expense of fabrication and subsequent excessive cost of operation due to inherent inefiiciency.

Another apparatus which is similar in principle, although totally differentiated in result, to that of the present invention is disclosed in the T. A. Herbert patent, No. 3,033,973. This apparatus is of the electric blanket type and permits better dwell times than those delineated above. Even so, this device is limited by the type of heater and electrical and thermal insulators employed therein to a maximum operational temperature of 1700 F. This limitation precludes its employment in processing of the exotic and refractory metals made possible by the present invention.

The present invention resides primarily in the novel utilization of a thermally stable electrical insulator which exhibits an extremely high rate of heat transfer through conduction while being, at the same time, metallurgically compatible with the refractory metal heaters employed therewith. This extremely high rate of heat transfer and metallurgical compatibility permits the attainment of extremely high temperatures with very brief heating-cooling cycles. This ability permits the requisite super alloy or refractory metal brazing temperature to be attained, but prevents temperature maintenance above the recrystallization temperature for time intervals which cause substantial degradation of the joint. That is, the present invention permits attainment, for the first time, of proper temperature-dwell time relationships permissive of achieving metallurgically and structurally satisfactory bonds between super alloy and refractory metals in an economical and relatively simple device capable of general utilization by any person having skill in the art.

Specifically, the device or" the present invention employs a thermally stable ceramo-metallic or refractory oxide type electrical insulator separator in combination with refractory metal heaters, the insulation being interposed between parallel, juxtapositioned ribbon heaters and the object workpiece. Such positioning permits the optimum current, heating-time cycle, and efficiency, but prevents direct electrical shorts or arcing.

In the preferred embodiment of the present invention the heaters are disposed upon either side of the workpiece with the insulators interposed therebetween, thus preventing electrical shorts as hereinabove stated. This configuration allows the workpiece to be, in effect, in physical contact with the heating media. Obviously, this method of heating possesses an intrinsically high rate of thermal conduction and efiiciency, thereby promoting very rapid heating rates. Further, since the heaters are adjacent to the workpiece, and the heat generated is confined by suitable means to the area occupied by the workpiece, only the areas immediately adjacent to the workpiece are heated, thereby substantially reducing the mass that is heated as well as substantially reducing residual heat. The combination of contact heating, high power inputs, and reduction of the mass that is to be heated results in a significant reduction of the dwell time within the recrystallization zone, and allows greater temperatures to be reached much more rapidly. These facts, in combination with the reduction of residual heat and a flow argon atmosphere further result in a very aburpt cooling cycle.

Ceramo-metallic insulator-separators are also employed to preclude deleterious chemical reactions experienced when the refractory metal heaters contact the thermal insulating means. This chemical reaction, if allowed, results in the deterioration of the heaters and subsequently in heater failure. Further, such insulators function as a heat distributing means, which is important because the heat generated by the heaters is localized due to their strip configuration. This distribution is effected by the positioning of the electrical insulators between the work and heating media, thereby uniformly conducting the generated heat over the entire area to be joined.

The novel combination of elements of the device of the present invention permits its efficient operation up to a maximum temperature of between 3900" F. to 4700 F. dependent upon the type of insulator used.

Ceramo-metallic or refractory oxide insulators according to the present invention are comprised of a refractory metal sheet, which in the preferred embodiment is molybdenum and an oxide coating. The metal sheet is slightly larger than the workpiece to permit beveling at the edges, thus precluding abrasions in the oxide coating and avoiding sharp edges at any point -of contact with the heaters. The refractory oxide insulator coating may be applied in either of two methods.

One method is to first paint a thin coat of levigated altunina on the refractory sheet in a slurry of 2 percent poly-vinyl-alcohol and 5 percent sodium silicate and subsequently bake the encapsulated sheet at 250 F. until dry. The sodium silicate improves adherence of the oxide to the sheets. A second identical coat is then applied 90 degrees to the strokes of the first coat. The second coat is followed by baking at 900 F. for one hour to drive off the major portion of the poly-vinylalcohol. The alumina alcohol-sodium silicate mixture should preferably be about the consistency of oil base paint. It should be noted that the insulators are not limited to molybdenum sheets or aluminum oxide for any refractory metal and that most oxides will suflice, however, beryllium oxide, magnesium oxide and thorium oxide have been found to be the most useful. A second method of application resides in flame spraying the oxide to the supporting molybdenum sheet and has been found to be extremely satisfactory. Obviously, the method employed is a matter of choice.

The salient object of the present invention is therefore to provide an apparatus operable within an extended range of temperatures up to approximately 4700 F. and capable of favorable dwell times, and a process to effect both brazing and diffusion joining of refractory metals and super alloys, thereby permitting fabrication of high temperature structural components, such as low density cellular core and honeycomb sandwich panel, while simultaneously precluding detrimental changes within the crystalline structure thereof.

It is another object of the invention to provide an electrical insulator possessing a very high heat transfer rate which comprises a refractory metal sheet encapsulated within a refractory oxide coating. Such coating permits contact heating of the workpiece thereby reducing the time required for the heating and cooling cycles.

It is still another object of the present invention to provide an apparatus of the class described which employs high temperature refractory metal resistance heaters.

It is yet another object to reduce high tooling and fabrication costs of presently available brazing apparatus.

Other and further features and object of this invention will become apparent to those skilled in the art in light of the following specification and drawings where- FIGURE 1 is an elevational view, partly in section of the device of the present invention in the preferred form;

FIGURE 2 is a pictorial, exploded view of the heaters and insulator-separators of the present invention revealing their typical relation to each other.

The preferred embodiment of the present invention, as shown generally in FIGURE 1, comprises a plurality of parallel juxtapositioned ribbon resistance heaters (best shown in detail in FIGURE 2), which heaters form heater banks 12 and 14, heater banks 12 and 14 being employed on diametrically opposed sides of a workpiece 16. Heater banks 12 and 14 are evenly spaced over the area to be processed and comprise a plurality of pure molybdenum ribbons, which in one preferred embodiment are of an inch wide, .002 of an inch thick and 14 inches long. Spaciing of ribbon elements 10 is maintained by doublers 18 and 20 positioned across the ribbons at their longitudinal extremities. It has been found that spot welding .005 inch columbium strips is satisfactory. Contact strips 22, 24, 26 and 28, are positioned on the ends of heater banks 12 and 14 respectively to transmit electrical power to the individual heater strips 10. This power transmission is accomplished by connection to two copper bus bars 30 and 32, which are subsequently attached to a DC. power supply shown schematically in FIGURE 1.

Electrical insulation between heater banks 12 and 14 and workpiece 16, as well as hereinafter described insulating bricks, is effected by four thin ceramo-metallic insulators 34, 36, 38, and 40 illustrated in FIGURE 1. Insulating bricks 42, which in one preferred configuration are 8 inches in length, 8 inches in width and 2 /2 inches in height, are employed to localize heat. Bricks 42 are bound together into slabs 44 by an adjustable strap 46 which may be of A286 a stainless steel alloy having a composition, in percent by weight, carbon .08; chromium 15.0; nickel 26.0; molybdenum 1.25; titanium 2.0; aluminum .25; vanadium 0.3; manganese 1.4; and remainder iron. Any stainless or cold rolled steel will, of course, suffice alloy. Bricks 42 are positioned as illustrated in FIGURE 1 above and below workpiece 16, and its associated insulators 34, 36, heater banks 12 and 14 and insulators 38, 40 respectively. Prior to the placement of edge bricks 48 and .50, a strap 52, which may be of titanium or the like, is positioned around the periphery of the workpiece to further prevent heat loss and to burn any oxygen present. Edge bricks 48 and 50, in one preferred embodiment 8 inches in length, /2 an inch in height and inch in width, are placed around the edges of workpiece 16 adjacent to strap 52. A small aperture (not shown) may be desirable for optically measuring the interisor temperatures at the workpiece.

All of the above described components are positioned on a suitable base 54 having spacers 56 thereon in order to position the workpiece in proper relation to the heaters. A suitable argon inlet 58 is mounted within base 54. Base 54 is subsequently provided with an air tight cover 60 thereby effectively forming a retort. Weights 62, as shown in FIGURE 1, may be employed to obtain any required pressure on workpiece 16 and are positioned upon upper insulating bricks 42. Other means of applying pressure may be employed such as differential pressure and/ or any suitable mechanical, hydraulic or pneumatic means.

The process of the present invention is initiated with a first purge of retort 60. It has been found to be desirable to employ a three cycle purge. Each cycle consists of vacuum pumping to less than 200 microns absolute pressure, followed by back filling with argon to ambient pressure. After purging is complete, the argon flow is adjusted to a steady 20 ft. /hr.

After purging the heating cycle is accomplished in two steps. The first step involves removal of an acrylic resin matrix in the powdered braze alloy and may be omitted if such a material is not employed. Where employed, this step consists of slowly increasing the temperature to approximately 1600" F. and holding at this temperature until all signs of smoke disappear. The second step resides primarily in rapidly increasing the power input to achieve a rate of heating of approximately 200 F. per minute. This rate is maintained until the recrystallization zone is reached. At this time, the input is suddenly increased, resulting in an extremely rapid increase to brazing temperature. This increase normally requires only thirty seconds. Power is then terminated in ord to minimize dwell time above the recrystallization temperature of the material to be brazed. The cooling rate is then recorded in one minute increments down to about 1100 F. In this process, employing the disclosed apparatus, the refractory materials being joined are within the recrystallization zone less than one minute, thus precluding detrimental recrystallization and grain growth.

For example, in a preferred embodiment wherein the material to be brazed is columbium FS-SZ alloy, the process employed is as follows: The material was set up in accordance with the prior description. The retort was purged three times as explained above. The temperature was raised gradually to 1600 F. and held until smoking disappeared. The power input was rapidly increased to 7 kilowatts, resulting in a rate of heating of about 200 F. per minute, which rate was maintained until a temperature of 2300" F. was achieved. The input was then increased suddenly to 8 kilowatts, resulting in a very rapid increase to the brazing temperature. This increase required only 30 seconds to complete. Power was then terminated, permitting a very rapid temperature falloff to below the recrystallization zone. The material was within this zone for less than 60 seconds total.

This is best illustrated in the following sandwich panel column compressive tests which verify that the present invention effects joining without significant recrystallization and grain growth with their corresponding loss of mechanical properties.

Room Temp. Room Temp. Panel Material Column Com- Ultimate Tensile Run No. Colurnbium Alloy pressive Strength Strength of Skin After Brazing Materials Before Brazing 125,000 p.s.i 97,000 p.s.i. 114,000 psi 95,000 psi. 95,000 p.s.i 83,600 psi. 142,000 psi 115,500 p.s.i.

Industry wide evaluations of brazed sandwich panels indicate that in the optimum condition, column compressive strengths should equal or slightly exceed the ultimate tensile strength of the skin material before brazing, thus indicating retention of full tensile properties in substantial absence of recrystallization and grain growth. As is ob vious from the above, this condition is fully satisfied in materials brazed by the process of the invention.

As thus described above in detail, the present invention for the first time provides a practical process and apparatus operative to braze or diffusion join refractory or super alloy materials without loss of desirable structural characteristics through intrinsic recrystallization and grain growth.

This is made possible by employment of a heat source comprising refractory metal strip resistance heaters. These heaters, due to their construction and ceramometallic insulation, may be positioned in very close proximity both to each other and to the workpiece being joined. This in turn effects a rapid heating cycle through extremely high heat transfer efficiency. In addition, such positioning substantially precludes extraneous heating of supporting and insulating components and thus deters any buildup of residual heat. This effectively decreases the cooling cycle, which in combination with the related rapid heating rates achievable by the invention appreciably reduces dwell time within the recrystallization and grain growth zone.

What is claimed is:

1. A device for joining refractory metal and super alloy materials without deleterious recrystallization occasioned by substantial periods within the materials recrystallization zone during the joining, comprising in com- 'bination:

(A) an enclosure means defining a selectively closeable workspace, said enclosure having means operative therewith to control the environment therewithin;

(B) first electrical insulation-separator means operatively associated with each major surface of an object workpiece to uniformly and rapidly distribute heat from a heat source directly to the object workpiece;

(C) heat source means positionable on each of said first electrical insulation-separator means on the portion thereof furthest removed from the workpiece, said heat source means comprising (1) a plurality of juxtapositioned, parallel ribbon resistance heater elements of a refractory metal material, and

(2) electrically conductive refractory metal contact elements positioned across and in intimate electrical contact with said plurality of ribbon resistance heaters to thereby transmit current to each of said ribbon elements,

(D) second electrical insulation separator means positionable on the portion of said heat source means furthest removed from said first electrical insulation separator means to uniformly and rapidly distribute heat from said heat source to the object workpiece; and

(E) thermal insulation means positionable on said second electrical insulation separator means on the portion thereof furthest removed from said heat source means and operative to confine heat generated by said heat source means to the immediate vicinity of the workpiece and substantially limiting the mass heated and resultant residual heat.

2. The device defined in claim 1 wherein each said first and second electrical insulation separator means comprises:

.a thin refractory metal sheet having on each side thereof a spray-like thin refractory oxide insulation coating intimate-1y bonded thereto.

3. A device for joining refractory metal and super alloy materials without deleterious recrystallization occasioned !by substantial periods within the materials recrystallization zone during the joining, comprising in combination:

(A) an enclosure means defining a selectively closeable workspace, said enclosure having means opera- 7 tive therewith to control the environment the-rewithin;

(B) first electrical insulation separator means operatively associated with each major surface of an object workpiece;

(C) heat source means positionable on each of said first electrical insulation separator means on the portion thereof furthest removed from the workpiece, said means comprising a resistance heating means having a plurality of resistance heating strips disposed in juxtapositioned parallel rows operatively interconnected to each other and to a selective source of electric current and operative to selectively heat the workpiece by conduction and radiation,

(D) second electrical insulation-separator means positionable on the portion of said heat source means furthest removed from said first electrical insulation-separator means,

(1) said first and second electrical insulation-separator means comprising (2) a metallic separator sheet in combination with and encompassed by an oxide coating, said means operating to preclude chemical reactions between said heat source means and a thermal insulation means and to evenly and rapidly distribute heat resultant from said heat source means and thus permit a minimal time within a materials recrystallization zone; and

(E) thermal insulation positionable on said second electrical insulation-separator means on the portion thereof furthest removed from said heat source means and operative to confine heat generated by said heat source means to the immediate vicinity of the workpiece and substantially limiting the mass heated and resultant residual heat.

4. The device as defined in claim 3 wherein said thermal insulation means is in part a spacing member operative to position the object workpiece within said enclosure means, and in part operative as a pressure transmitting member to provide at least a portion of a predetermined pressure to the object workpiece.

5. A high temperature apparatus wherein joining processes of various pressure-temperature relationships may be performed on refractory and super alloy metals under controlled environmental conditions, comprising in com bination:

(A) enclosure means defining a floor, sidewalls and roof and operative to contain a specific environment free of contamination from outside sources,

said enclosure incorporating means therewith operative to selectively purge said enclosure and inject and maintain a given environment therewithin;

(B) first thermal insulating means positioned within said enclosure means in spaced relation to said sidewalls,

said means comprising individual thermal insulation brick-like elements incorporated into a slab variable in configuration;

(C) a first metallic separator means,

(1) said means comprising a refractory metal sheet encapsulated by a ceramo-metallic oxide coating and thus functionable as an electrically insulating means,

(2) said means positioned upon said first thermal insulating means and operative to prevent chemical reactions between a heat source and said thermal insulation;

(D) a first heater means operatively associated with an electrical power source,

(1) said means being defined by a plurality of electrical resistance heater strips,

(2) said strips disposed in interconnected juxtapositioned rows defining a heater bank,

(3) each said strip comprising a thin refractory metal element,

(4) said bank positioned adjacent to and in contact with said first separator means;

(E) a second metallic separator means,

(1) said means comprising a refractory metal sheet encapsulated by a ceramo-metallic oxide coating and thus operable as an electrical insulating means,

(2) said means adjacent to and in planar contact with said first heater means on the portion thereof furthest removed from said first separator means,

(3) said means uniformly and substantially instantaneously distributing heat generated upon activation of said heat source to an object workpiece in intimate contact with said second separator means;

(F) means for radiating heat and scavaging contaminates from the environment immediately adjacent an object workpiece,

said means adjustably positionable around the periphery of the workpiece;

(G) a third metallic separator means,

(1) said means comprising a refractory metal sheet encapsulated by a ceramo-metallic oxide coating and operable as an electrical insulating means for a second heater means,

(2) said means adjacent to and in planar contact with a second heater means on the face thereof in closest proximity to the object workpiece,

(3) said means uniformly and substantially instantaneously transmitting and distributing heat generated upon activation of a second heater means to the object workpiece and in intimate planar contact therewith;

(H) a second heater means operatively associated with an electrical power source,

(1) said means comprising a plurality of electrical resistance heater strips,

(2) said strips disposed in interconnected juxtapositioned rows defining a heater bank,

(3) each said strip comprising a thin refractory metal element;

(I) a fourth metallic separator means,

(1) said means comprising a refractory metal sheet encapsulated by a ceramo-metallic oxide coating and serving to electrically insulate said second heater means on one face thereof,

(2) said means adjacent to and in planar contact with said second heater means on the face thereof furthest removed from the object workpiece,

(3) said means operative to prevent chemical reactions between said second heater means and a thermal insulator; and

(J) second thermal insulating means,

(1) said means comprising individual thermal insulating brick-like elements incorporated into a variable configuration slab,

(2) said means serving to confine and direct radiant heat from said heater means to the object workpiece and thus obviating any unnecessary and unproductive heat buildup or residual heat within said enclosure.

References Cited by the Examiner UNITED STATES PATENTS 3,053,969 9/1962 Kerr et al 2l9-117 3,087,046 4/1963 Mellinger 219- 3,202,792 8/1965 Bukata 219-85 RICHARD M. WOOD, Primary Examiner.

B. A. STEIN, Assistant Examiner. 

1. A DEVICE FOR JOINING REFRACTORY METAL AND SUPER ALLOY MATERIALS WITHOUT DELETERIOUS RECRYSTALLIZATION OCCASIONED BY SUBSTANTIAL PERIODS WITHIN THE MATERIALS RECRYSTALLIZATION ZONE DURING THE JOINING, COMPRISING IN COMBINATION: (A) AN ENCLOSURE MEANS DEFINING A SELECTIVELY CLOSEABLE WORKSPACE, SAID ENCLOSURE HAVING MEANS OPERATIVE THEREWITH TO CONTROL THE ENVIRONMENT THEREWITHIN; (B) FIRST ELETRICAL INSULATION-SEPARATOR MEANS OPERATIVELY ASSOCIATED WITH EACH MAJOR SURFACE OF AN OBJECT WORKPIECE TO UNIFORMLY AND RAPIDLY DISTRIBUTE HEAT FROM A HEAT SOURCE DIRECTLY TO THE OBJECT WORKPIECE; (C) HEAT SOURCE MEANS POSITIONABLE ON EACH OF SAID FIRST ELECTRICAL INSULATION-SEPARATOR MEANS ON THE PORTION THEREOF FURTHEST REMOVED FROM THE WORKPIECE, SAID HEAT SOURCE MEANS COMPRISING (1) A PLURALITY OF JUXTAPOSITIONED, PARALLEL RIBBON RESISTANCE HEATER ELEMENTS OF A REFRACTORY METAL MATERIAL, AND (2) ELECTRICALLY CONDUCTIVE REFRACTORY METAL CONTACT ELEMENTS POSITIONED ACROSS AND IN INTIMATE ELECTRICAL CONTACT WITH SAID PLURALITY OF RIBBON RESISTANCE HEATERS TO THEREBY TRANSMIT CURRENT TO EACH OF SAID RIBBON ELEMENTS, (D) SECOND ELECTRICAL INSULATION SEPARATOR MEANS POSITIONABLE ON THE PORTION OF SAID HEAT SOURCE MEANS FURTHEST REMOVED FROM SAID FIRST ELECTRICAL INSULATION SEPARATOR MEANS TO UNIFORMLY AND RAPIDLY DISTRIBUTE HEAT FROM SAID HEAT SOURCE TO THE OBJECT WORKPIECE; AND (E) THERMAL INSULATION MEANS POSITIONABLE ON SAID SECOND ELECTRICAL INSULATION SEPARATOR MEANS ON THE PORTION THEREOF FURTHEST REMOVED FROM SAID HEAT SOURCE MEANS AND OPERATIVE TO CONFINE HEAT GENERATED BY SAID HEAT SOURCE MEANS TO THE IMMEDIATE VICINITY OF THE WORKPIECE AND SUBSTANTIALLY LIMITING THE MASS HEATED AND RESULTANT RESIDUAL HEAT. 